A new approach has been used both in the laboratory and in field trials, to study the development and performance of special vulcanizates used in sealing applications. The use of conventional, monomeric fungicides has been replaced by polymeric organotin compounds covulcanized into the rubber network. Polychloroprene, acrylonitrile-butadiene rubber and ethylene-propylene terpolymer are three rubbers commonly used in military equipment and these have been shown to covulcanize with tributyltin acrylate in the presence of peroxides to give vulcanizates with a powerful and long-lasting fungicidal action. These rubbers have been evaluated by the use of O-rings to form sealed cavities with ten different materials (two metals, seven plastics and glass), in specially designed assemblies. Trials held over 3 years in adverse environments have confirmed that the rubbers are compatible with the sheet materials, can be formed into efficient sealing products that will prevent fungal growth, have good performance characteristics, and do not degrade the optical qualities of transparent materials. These rubbers should have applications in military, industrial and commercial equipment.